Saturday, March 12, 2016

Apple iMac 21.5 inch – battery testing and replacement – Resetting the RAM – Resetting system controller – Safe mode boot up – Desktop PC repair and service

Category: Desktop PC Repair and Service 

Contents of this article 

  • Resetting the RAM Parameters 
  • Safe mode Boot up  
  • Coin battery testing and replacement 




Apple iMac 21.5 inch

Common Reset Procedures
Power On Self Test (POST)

Intel-based Mac computers such as the iMac rely on a combination of tones and blinking LEDs to display Power On Self Test (POST) error codes.
# If the computer detects out-of-specification or no SDRAM, the screen will remain black but the computer will beep. This error condition may be due to physically damaged RAM, installing an incorrect type of RAM, or not having RAM installed.
# Some RAM may appear to pass the Power-On-Self-Test (POST) but still cannot be used by the operating system. In this case, the computer will display a gray screen, sound three tones and repeat tones until computer is turned off.
# The solution to both of these situations is to first re-seat memory and test computer again.
If memory fails the POST test again, remove all installed memory and test by installing one by one each memory module that has been verified to work correctly on another system (i.e., “known-good” memory) or order new memory.
# A sequence of tones heard at startup or a no video symptom may also be fixed by temporarily removing/replacing the backup battery.
Resetting the System Management Controller (SMC)
The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac:
1. From Apple menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off).
2. Unplug all cables from computer, including power cord.
3. Press and hold the power button for 5 seconds.
4. Release the power button.
5. Attach the computer’s power cable, making sure power button is not being pressed.
6. Press the power button to turn on the computer.
Resetting Parameter RAM (PRAM)
PRAM stores certain system and device settings in a location that Mac OS X can access quickly. Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:
1. Shut down the computer.
2. Locate the following keys on keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in Step 4.
Note: If the keyboard does not have an Option key, use the Alt key instead.
3. Press power button.
4. Immediately press and hold Command-Option-P-R keys.
Important: You must press this key combination before the gray screen appears.
5. Hold down keys until the computer restarts, and you hear the startup chime a second time.
6. Release keys.
Starting Up in Safe Mode
A Safe Boot is a special way to start Mac OS X when troubleshooting. Starting up into Safe Mode does several things that can help resolve software or directory issues that may exist on the startup volume. To start up in Safe Mode:
1. Make sure computer is shut down.
2. Press power button.
3. Immediately after you hear the startup tone, press and hold Shift key.
Note: The Shift key should be held as soon as possible after startup tone but not before.
4. Release Shift key when you see the screen with the gray Apple and progress indicator (looks like a spinning gear). Note that booting into Safe Mode will take longer than a normal startup. During startup, the words ”Safe Boot” appear on the Mac OS X startup screen and a gray progress bar is displayed on bottom of window (since Mac OS X 10.6).
5. To leave Safe Mode, restart computer normally, without holding down any keys during
startup.
Removing the Battery, Measuring DC Voltage
1. Shut down and unplug the computer. Allow several minutes for power supply to discharge.
2. Remove logic board.
3. Remove coin battery for 1-2 minutes. The coin battery is located on the back side of the logic board (see graphic below).

4. Measure DC voltage on battery touching battery with red probe, and grounding with black
probe. If voltage is 2.7v or less, replace battery.
5. Reinstall the battery and logic board. This will reset the logic board.
6. Power on computer.
7. If computer boots, check for and install all software and firmware updates.